One of the more common designs for automotive front suspensions is known as the McPherson strut suspension. As with all suspensions for motor vehicle applications, the components used are selected to provide a compromise between ideal suspension function and ride comfort. For this reason, rubber bushing elements are commonly used at some suspension pivot points, while various types of ball joints are used at others.
McPherson strut suspensions utilize a control arm that is pivotally attached to the chassis, with rotation about one axis, to allow vertical movement of the suspension and tire. A strut assembly, commonly referred to as a strut, and consisting of a shock absorber mounted inside a coil spring, is rigidly attached to a steerable spindle, to which a rotating hub and the vehicle wheel are attached. The spindle can be bolted to the bottom of the strut assembly, or may be integrated with the strut.
McPherson strut suspensions commonly use cylindrical rubber bushings at the two inner pivot points of the suspension control arm, pressed into two respective female bores within the control arm, and retained to the vehicle chassis with a bolt or pin at each location. Suspension arm pivot bushings allow rotation about one axis, while providing compliant attachment of the arm to the chassis to dampen vibrations and shock loads.
A single ball joint is normally used at the outer end of the control arm to pivotally attach the suspension strut to the control arm. The common ball joint design utilizes a ball joint pin, consisting of a metal sphere and cylindrical pin, installed into a plastic spherical female socket, and sealed with a rubber boot and lubricant. A ball joint allows rotation about three axes, with the center of rotation occurring at the center of the sphere of the ball joint pin. Rotation about the ball joint pin centerline is required to accommodate the steering angle, and is not limited by the geometry of the ball joint. Rotation in the other two directions is limited by the mechanics of the ball joint, with a total angular deflection normally between 40 and 60 degrees.
The attachment point connecting the strut assembly to the vehicle chassis is commonly referred to as the upper strut mount, or upper strut bearing. As with the ball joint at the bottom of the strut, the upper strut mount must allow rotation about all 3 axes. A bearing or bushing is normally used to allow rotation about the strut centerline axis. Rotation about the other two axes is much less at the top of the strut than is required at the bottom of the strut, due to the geometric configuration of the McPherson strut design. While the ball joint at the bottom might allow a total angular deflection of 40 degrees of more, the upper strut mount requires a total angular deflection from the relative horizontal plane of less than 20 degrees, and generally less than 10 degrees.
Because the suspension loads from the vehicle wheel are transferred directly through the spindle into the strut assembly, it is necessary to utilize a compliant upper strut mount to minimize the shock loads and vibration that are transferred to the vehicle chassis. Because the upper strut mount must also allow limited rotation about the two relative horizontal axes, a large rubber bushing element, or multiple bushing elements, is commonly used, which allows this rotation and provides both vertical and lateral compliance. While the vertical compliance provides the damping qualities that ensure good ride quality, the lateral compliance allows large amounts of lateral deflection under heavy cornering loads, which directly leads to loss of tire camber angle, loss of tire grip, and loss of cornering performance.
There are many different versions of strut suspension upper strut mounts used in production vehicles. Some designs utilize one integrated, molded rubber bushing that carries both upward and downward vertical loads, in addition to lateral loads. Other designs utilize separate upper and lower bushings to carry the upward vertical, downward vertical, and lateral loads. Still other designs utilize multiple bushings. The common characteristic of these designs is that there is sufficient compliance based on the geometry, thickness, and durometer of the rubber bushings to allow the required rotation about the two relative horizontal axes. Another common feature of upper strut mounts is that the inner housing has a central hole, through which the strut shock absorber piston rod is mounted and secured with a threaded nut.
For racing car applications, the rubber control arm bushings and the upper strut mounts are replaced with spherical bearings and/or plain bearings that have negligible compliance and negligible deflection, even under extremely high loads. Both spherical bearings and plain bearings may be metal-to-metal designs, or they may use thin non-metallic liners to reduce friction and wear and to extend life. Sometimes rigid plastic bushings may be used.
Teflon-lined spherical bearings are commonly used for racing applications to replace the upper strut mount, which provides the required rotation about all 3 axes. One negative aspect of this change is a tremendous increase in the shock loading and vibration that is transferred to the chassis. This level of deterioration in ride quality is normally unacceptable for a street-driven vehicle. However, this change eliminates the lateral deflection that normally occurs at the top of the strut due to the deflection of the upper strut mount, ensuring that optimum tire camber angle is maintained under heavy cornering loads.
Spherical metal bearings and plain metal bearings used in automotive racing applications are not generally designed to carry the impact loads that occur during street driving. Furthermore, metal bearings of all types are subject to wear, damage and noise, and require relatively frequent maintenance and replacement. These types of bearings are often unsealed, and exposure to water and road grit during street driving will further contribute to rapid wear and deterioration.
There is therefore a need for an upper strut mount that combines the beneficial aspects of rubber control arms bushing and spherical bearings. The following sections describe and claim applicant's inventive upper strut mount.